1. Field of the Invention
The present invention relates to an automatic transmission apparatus and a straddle-type vehicle including the automatic transmission apparatus.
2. Description of the Related Art
Automatic transmission apparatuses equipped with a multi-geared type transmission mechanism having a dog clutch, a clutch, and one or two actuators for driving the clutches and the transmission mechanism are conventionally known. In the automatic transmission apparatus, a control device such as an Electronic Control Unit (ECU) outputs a gear shifting command according to the driving condition of a straddle-type vehicle, or the rider allows the gear shifting command to output by operating a shift switch or the like, so that the actuator is driven. The actuator performs a series of operations, such as causing the clutch to disengage, subsequently changing the gear position of the transmission mechanism, and thereafter causing the clutch to engage.
When the clutch is disengaged, a rotational speed difference arises between a drive side rotator and a driven side rotator of the clutch. When the rotational speed difference is large in the clutch just before the engagement, a shock occurs at the time of engagement of the clutch. JP 2006-170225 discloses a technique for reducing such a shock. The automatic transmission apparatus described in JP 2006-170225 detects a rotational speed difference between the drive side rotator and the driven side rotator and adjusts the engagement speed of the clutch according to the rotational speed difference.
However, a shock also occurs when disengaging the clutch. The details are as follows. The clutch is engaged before shifting gears. Therefore, torque is transmitted from the drive side rotator to the driven side rotator, and mechanical power is transmitted to the driving wheel. However, when the clutch is disengaged at the time of shifting gears, the torque transmitted to the driven side rotator is lost. As a consequence, the mechanical power is no longer transmitted to the driving wheel. If the torque is abruptly lost, the mechanical power transmitted to the driving wheel is instantly lost. Such an abrupt loss of the drive force can be a shock that impairs the riding comfort for the rider. Although the above-mentioned conventional technique aims at reducing the shock at the time of engaging the clutch, it does not address a shock at the time of disengaging the clutch.